Presently known metal vapour lasers usually operate in gaseous electrical discharges at pressures below 1 bar. Metal, or a metallic compound, is contained in the laser discharge tube. The metal or metal compound is heated to a temperature at which the metal vapour pressure is sufficient to support laser action when the vapour is suitably pumped to establish a population inversion. One disadvantage of presently known metal vapour lasers is that the time required to achieve the operating temperature (termed the "warm-up" time) for laser action can be unacceptably long. Another disadvantage is that a laser tube containing free metal or a metallic compound cannot be moved, tilted or operated vertically without some provision being made to retain the solid or liquid in position along the tube.